The present invention relates generally to the testing and measurement of digital media devices, and specifically to a method and apparatus for evaluating the performance of systems that autodetect the coding format of incoming data streams.
Digital-media devices are required to interpret data streams that may be delivered in various coding formats. Sources of coded audio and video data include satellite TV receivers, game consoles, CD and DVD players, personal computers, etc. In the case of coded digital audio, devices are typically designed to communicate interchangeably through an IEC 60958 interface. This interface was originally designed to carry stereo linear PCM data, however IEC 61937 specifies how multi-channel compressed audio formats (e.g. AC-3, DTS, etc.) may be transmitted on the IEC 60598 interface.
FIG. 1 illustrates a typical multi-source digital-audio receiver that performs autodetection of the coding format of incoming audio data. The coded signal sources 010a-010d provide audio data in several distinct coding formats. A source is connected via the input selector 015 to the IEC 60958 receiver 020. The IEC 60958 receiver 020 feeds the coded audio data to an autodetection system 030. The autodetection system 030 decides the coding format of the incoming data and makes a format selection 035. This instructs a decoder selector 040 to switch the incoming data stream to either a decoder bypass 051 in the case of linear PCM data, or in the case of compressed audio coding formats to the appropriate decoder 050a-050d. After the decoder section output 060 there may be other digital signal processing, mixing or amplification.
If an autodetection system as illustrated in the previous example fails to correctly (or quickly) determine the coding format of the incoming data, the decoder circuitry may fail to produce audio. Worse yet autodetection failure may create audible and obnoxious decoding artifacts in an attempt to decode data in the incorrect format. It is a goal when designing an autodetection system that coding formats will be quickly and accurately detected so that drop outs or audible artifacts in the decoded audio output are absolutely minimized.
Autodetection systems such as described above commonly identify the format of incoming data by detecting synchronization patterns in the data stream which are designed to be unlikely to appear in the actual program material, as well as other flags. There are a wide variety of methods for autodetection, and a general discussion of these is beyond the scope of this disclosure. U.S. Pat. No. 6,205,223 discloses systems and methods for autodetection.
Existing equipment developed for testing digital-audio decoders does not address the need for thorough and repeatable measurements for the evaluation of coding format autodetection systems. There are test systems commercially available which can produce coded test signals in order to perform standard audio measurements on devices which include decoder circuits (e.g. dynamic range, THD+N, etc.) These existing decoder testing systems are not equipped to test autodetection algorithms.
The German firm Rohde and Schwarz offers the Audio Analyzer UPL Option UPL-B23. It is capable of delivering coded test signals in several formats, but it uses test waveforms designed to avoid the creation of audible artifacts when switching between test signals. This is done in order to perform the standard audio measurements without disruption, but also demonstrates that the system is severely limited for testing the failure modes of autodetection algorithms.
Audio Precision manufactures the 2700 Series Audio Analyzers, and the OPT-2711 Dolby Digital Option allows real-time generation of AC-3 encoded test signals for test and measurement purposes. The OPT-2711 system also lacks the capabilities necessary for properly testing autodetection algorithms. Specifically it lacks the capability to assess the performance of autodetection systems under variance in signal timing and source characteristics, and their behavior when encountering a change of coding format.
There are commercially available DVDs which contain audio test signals in various coding formats. Rohde and Schwarz offers a set of Professional Test DVDs containing test signals in various coding formats. These types of products in combination with conventional digital-audio test equipment can provide a limited tool to address the requirements of testing autodetection algorithms. However using a pre-sequenced test disc is no substitute for a programmable autodetection testing system. While playing a test DVD there can be no variance of source-specific characteristics (e.g. signal level, jitter, etc.) that might be encountered in a realistic multi-source application.
U.S. Pat. No. 6,138,051 describes a system for testing audio decoders, in the form of encoded bitstreams which will produce an audible signal at the decoder output when the decoder fails. However that disclosure does not at all address the needs of testing autodetection algorithms.
The lack of test equipment designed for precise and repeatable evaluations of autodetection algorithms has led engineers in the field to come up with their own improvised solutions. A typical scenario involves the use of multiple CDs or DVDs, with program material encoded in various audio formats. The discs are interchanged in an appropriate player. The test engineer will manually select tracks and thereby choose between different combinations of test signals and coding formats. Using this homemade method an engineer may be able to detect certain design flaws in an autodetection algorithm, but the process is tedious and very imprecise.
There are substantial variations in the behavior of different audio sources when changing disc tracks. For example, when many DVD and CD players change tracks, the player's IEC 60958 output interface emits zero valued samples until new audio data is available. Other sources emit special code words called “pause bursts” as described in U.S. Pat. No. 6,076,062 between intervals of actual audio playback. Still other audio sources will completely silence their digital outputs, producing no digital signals at all.
There is significant variation in the behavior of coded audio sources even within each of these three groups. For sources which emit “digital black” between tracks, some will maintain the interface status bits and sample rate associated with the last track played while others will switch to a default sample rate and/or set of status bits. Also differences in output voltage and jitter levels between the IEC 60958 output interfaces of coded audio sources can lead to some receivers having difficulty locking onto the data stream. This type of failure can interfere with the timely passing of data to an autodetection algorithm.
There is no existing digital audio test system which can detect the effect of these functional differences on the performance of an autodetection system.
In summary the current art of autodetection algorithm testing is severely limited in the ability to reproduce the timing and variety of playback and source switching events. At best the improvised methods currently in use could be described as a stochastic test procedure, with the major drawbacks of operational tedium and a lack of flexibility, repeatability and precision. At worst engineers using this method of testing may completely miss operating conditions which could produce audio dropouts, audible artifacts or even “lock-up” of the autodetection algorithm(s) resulting from a transition between coding formats.
There are a few devices outside of the field of test and measurement which superficially include a few of the desired features of an autodetection testing system. U.S. Pat. No. 6,629,197 discloses a system which programmably emulates some operational characteristics of a CD changer, delivers the digital-audio contents of “virtual CD-ROMs” and responds to external control signals. A product built using the aforementioned invention merely achieves compatibility with existing home theater devices by emulating a user-selected changer. However the system disclosed therein fails to address the needs of test sequencing, synchronization, and signal analysis for an automated test application.